Why Boron Nitride is such a Selective Catalyst for the Oxidative Dehydrogenation of Propane

Abstract

Boron‐containing materials, and in particular boron nitride, have recently been identified as highly selective catalysts for the oxidative dehydrogenation of alkanes such as propane. To date, no mechanism exists that can explain both the unprecedented selectivity, the observed surface oxyfunctionalization, and the peculiar kinetic features of this reaction. We combine catalytic activity measurements with quantum chemical calculations to put forward a bold new hypothesis. We argue that the remarkable product distribution can be rationalized by a combination of surface‐mediated formation of radicals over metastable sites, and their sequential propagation in the gas phase. Based on known radical propagation steps, we quantitatively describe the oxygen pressure‐dependent relative formation of the main product propylene and by‐product ethylene. Free radical intermediates most likely differentiate this catalytic system from less selective vanadium‐based catalysts.

Highly Selective Carbon‐Supported Boron for Oxidative Dehydrogenation of Propane

“…Notably, h‐BN does not appear to deactivate but features an induction period for optimal catalytic performance, suggesting that the oxidized/hydrolyzed boron layer provides the active species [1,5,8,11] . This is in agreement with previous quantum chemical calculations that suggested clustering of boron oxide/hydroxide species are required for ODH [10] …”

“…Structured carbon surfaces are often functionalized by chemical treatments to induce thermal stability under oxidative conditions (Figure 1). [6–14] This is done by inducing surface functional groups, such as −OH and −COOH, with an oxidizing treatment ( e. g . nitric acid) [6–14] .…”

“… [6–14] This is done by inducing surface functional groups, such as −OH and −COOH, with an oxidizing treatment ( e. g . nitric acid) [6–14] . These functionalities are anchoring sites for boron precursors, thus giving the ability for carbon to support greater quantities of active sites, but also restricting the surface mobility of the species.…”

“…85 % at 9 % conversion), significantly outperforming previous state‐of‐the‐art silica‐supported vanadium oxide ODH catalysts (61 % at 9 % conversion) [1] . The enhanced performance of h‐BN for ODH is attributed to an amorphous oxidized/hydrolyzed boron layer [B 2 (OH) 2x O 3‐x , x=0–3] which resides on the BN sheets [2,7–10] . Solid‐state NMR spectroscopy and other surface characterization techniques have shown that the amorphous oxidized/hydrolyzed boron layer is primarily composed of 3‐coordinate boron oxide/hydroxide species (B 2 (OH) 2x O 3‐x , x=0–3) that form under reaction conditions [2,9,11,12] .…”

High‐Density Lewis Acid Sites in Porous Single‐Crystalline Monoliths to Enhance Propane Dehydrogenation at Reduced Temperatures

“…There are mainly two types of approaches including oxidative and non‐oxidative propane dehydrogenation to propylene. For oxidative dehydrogenation, although the propane conversion is high the propylene selectivity is normally below 80 %, which is mainly due to the over‐oxidation and cracking of propane [4–7] . In contrast, the non‐oxidative propane dehydrogenation with noble‐metal catalyst like PtSn x gives the conversion of ≈50 % and the propylene selectivity of ≈90 % at ≈600 °C [8–13] .…”